Felsite Dykes Research Articles

Geochemical characterization of altered rocks and element dispersion around an abandoned mining site in an arid zone: A case study of Fatira gold mine area, Eastern Desert, Egypt

Environmental impacts related to mining are widespread due to poor waste management. Fatira abandoned gold mine area in Egypt was studied for the geochemical/mineralogical properties of alterations haloes, mine tailings, stream sediments under dry conditions and impacts to nearby environment. Results of this study indicated that stream sediments and soil-tailings mixture were derived from felsic sources, while tailings, weathered and old tailings were obtained from intermediate-felsic sources. The Pb load was much higher in weathered rocks, soil-tailings mixture and old tailings, while metals like As, Co, Cr, Cu, Ni, V, and Zn show elevated values in weathered rocks nearby Fatira mine area and in downstream sediments. The tailings have high average concentrations of heavy elements, Sr (439), Zn (389), Cu (310), and Cr (50) (in mg/kg), higher than the toxicological standard values for contaminants indicating adverse effects to the neighbouring environment, whereas, soil-tailings mixture have higher As (263), Pb (56) and Al2O3 (17 wt%) reflecting the metavolcanics and granite composition of country rocks. At lower depths of tailings, escalated concentrations of Al2O3, K2O, Na2O, and Fe2O3, and little CaO, MgO, As, Pb, Zn, Cu, and Ni are indicate variable input from intermediate rocks, grey granites, felsite dykes and pink granites. Chemically altered granites showed replacement of hornblende into actinolite, calcite, pitted ilmenite and in-situ formation of Fe-oxides/hydroxides. Low hydraulic conductivity of the rocks increased the intensity of weathering. Gypsum and goethite are recorded due to repetitive sulfides decomposition. The element enrichment that exceeds the standard toxicological values in tailings, soil-tailings mixture, weathered rocks and stream sediments have severe effects on health. Integrating the chemistry, mineralogy and microtextural analyses in the study area is required for accurate geoenvironmental forecasting that facilitate mine rehabilitation with an effective management plan.

Characteristics and nature of gold-bearing fluids in Fatira area, North Eastern Desert of Egypt: possible transition from intrusion-related to orogenic deposits

The gold lode in Fatira area is represented in three modes of occurrence: (1) altered felsite dykes, (2) massive quartz veins that are cutting through the hosting granitic rocks, and (3) siliceous-hybrid shear zone which is trending in NNW-SSE direction and separating between metavolcanic and granitic rocks. Metavolcanic rocks are described by meta-andesite and meta-dolerite varieties; they exhibit calc-alkaline magma type related to island arc system. However, granitic rocks are of I-type granodiorite composition with calc-alkaline affinity developed in convergent plate environment (island arc/active continental margins) during the late stage of orogeny. The wallrock alteration around the ore body includes propylitization, sericitization, and silicification facies. The ore minerals were observed as disseminated sulfides (pyrite, sphalerite, chalcopyrite, and covellite). Goethite, jarosite, and ilmenite are the main iron-rich mineral phase as well as malachite and rutile. Gold was detected by microprobe analysis within the internal composition of pyrite and/or jarosite. Two groups of fluid inclusions could be recognized in samples from different mineralized zones: (a) aqueous fluid inclusions (H2O–NaCl/KCl) and (b) aqueous-carbonic fluid inclusions (H2O–CO2–NaCl/KCl). Isochore construction from fluid inclusion data, combined with chlorite thermometry from alteration zone, revealed that Fatira gold deposits were formed at temperature range of 275–297 °C and pressure between 0.2 and 1.1 kb). The Fatira gold deposits exhibit similar features of both intrusion-related and orogenic deposits. Hence, the initial ore-bearing fluid was proposed to have originated from magmatic source at deeper levels related to the granitic emplacement (5–10 kb); moreover, the granitic batholith acted as an important vehicle for driving of the metamorphic hydrothermal currents and probably as a cause for the metamorphic process itself. Then, the migration of gold was taken place in form of bi-sulfide complexes and the final deposition occurred through fluid uplifting consequent with a possibility of mixing with meteoric water at shallow depths, pressure/temperature drop, and fluid/wallrock interactions.

Geology, geochronology, and fluid inclusion studies of the Xiaorequanzi volcanogenic massive sulphide Cu–Zn deposit in the East Tianshan Terrane, China

The Xiaorequanzi Cu–Zn deposit is in the westernmost part of East Tianshan Terrane in northwestern China. The deposit is unique in the region being a volcanogenic massive sulphide (VMS) deposit located near a zone (or belt) containing giant late Paleozoic porphyry Cu deposits. Aiming to better understand the genesis of the mineral deposits in the terrane and their tectonic setting, we report our findings of detailed studies on fluid inclusion microthermometry, Re–Os dating of chalcopyrite from the massive ore, and U–Pb dating of zircons from the host volcanic rocks. There are two sulphide stages with early pyrite succeeded by chalcopyrite–sphalerite, which are hydrothermally overprinted and supergene enriched. The hydrothermal overprinting is characterised by quartz–sulphide veins crossed by carbonate-rich quartz veins. Quartz from the chalcopyrite–sphalerite stage is characterised by primary fluid inclusions containing H2O–NaCl(–CO2) and homogenise at 228–392 °C with a salinity of 2.2–13.3 wt.% NaCl equiv. Secondary fluid inclusions related to the hydrothermal overprinting homogenise at 170–205 °C with a salinity of 2.7–12.1 wt.% NaCl equiv. Fluid inclusions in the quartz–sulphide stage of the hydrothermal overprinting contain H2O–NaCl with homogenisation temperatures of 164–281 °C and salinities in ranging from 2.9 to 12.4 wt.% NaCl equiv. Fluid inclusion in the quartz–calcite stage contain H2O–NaCl with homogenisation temperatures of 122–204 °C with salinities of 1.4–12.4 wt.% NaCl equiv. These characteristics are like those of the secondary fluid inclusions in the VMS mineralisation. Combining these findings with H–O isotopic data from previous studies, we propose that the primary mineralising fluid is magmatic in origin. Tuff hosting the mineralisation yields a SHRIMP U–Pb zircon age of 352 ± 5 Ma, which is interpreted as the age of the tuff, and a porphyritic felsite dyke intruding the tuff yields a SHRIMP U–Pb zircon date of 345 ± 6 Ma, interpreted as the emplacement age of the dyke. Chalcopyrite from the main orebody at Xiaorequanzi yields a Re–Os isochron age of 336 ± 13 Ma with an initial 187Os/188Os ratio of 0.25 ± 0.55 (MSWD = 12). Given that the VMS deposit is a syngenetic deposit, we regard the upper ca. 349 Ma limit of the Re–Os date as the approximate age of the chalcopyrite. The three dates are the same within error, and the upper limit of the Re–Os date of ca. 349 is taken as the age of the volcanic, dyke, and mineralisation. The volcanic rocks around the Xiaorequanzi deposit have been previously classified as calc–alkaline to high-K calc–alkaline enriched in large-ion lithophile elements and depleted in high-field-strength elements, which are characteristics indicative of a forearc setting. It is suggested that VMS mineralisation formed in a forearc setting related to the north-directed subduction of the Palaeo-Kangguer or North Tianshan oceanic plates.

REE mineral chemistry and the nature of REE mineralization: A study from felsite dykes of Phulan area, Siwana Ring Complex, Rajasthan, India

Neo-Proterozoic Siwana Ring Complex (SRC) comprises per alkaline rocks of Malani Igneous Suite, viz., rhyolite, granite, and late phase microgranite and felsite dykes. Phulan area, lying at the northeastern margin of SRC exposes a small body of rhyolite (<1.0 km2) containing feldspar + quartz + aegirine + rebeckite and is cut by dykes of felsite. These felsite dykes have a general NNW–SSE trend and vary from 60–200 m long and 0.10–2.50 m wide. These dykes are composed of quartz, alkali feldspar, aegirine and opaques. These felsite dykes were sampled and analyzed using inductively coupled plasma methods. Of special significance is the enrichment of trace elements and rare earth elements (REE) in the felsite dykes. These include up to 1.17% Ce, 0.6% La, 0.8% Y, 0.12% Dy, 169.25 ppm U, 571 ppm Th, 1385 ppm Nb, 9944 ppm Zr. These dykes are peralkaline in nature and show negative europium (Eu) anomaly. In this study, the authors attempted to characterize REE bearing phases of the felsite dykes of Phulan area, SRC with respect to their geneiss. REE bearing phases identified in felsite dykes are monazite, bastnaesite, parisite, eudiyalite, allanite, perreierite and tritomite. Monazite, perreierite, allanite and tritomite are mostly found to be of magmatic in origin whereas bastnaesite, parasite and eudiyalytes occur both as magmatic and as well as of hydrothermal types. Magmatic REE minerals are mostly formed during crystallization of REE rich magma. In felsite dykes, Zr/Hf ratio varies from 23 to 31 and Nd/Ta ratio ranges from 7 to 44. These two ratios are positively correlated and indicators of hydrothermal fluid influx.

Geochemistry, geochronology and petrogenesis of Maya Block granitoids and dykes from the Chicxulub Impact Crater, Gulf of México: Implications for the assembly of Pangea

The Late Paleozoic tectono–magmatic history and basement of the Maya block are poorly understood due to the lack of exposures of coeval magmatic rocks in the region. Recently, IODP–ICDP Expedition 364 recovered drill core samples at borehole M0077A from the peak ring of the Chicxulub impact crater, offshore of the Yucatán peninsula in the Gulf of México, have been studied comprehensively. In the lowermost ~600 m of the drill core, impact–deformed granitoids, and minor felsite and dolerite dykes are intercalated with impact melts and breccias. Zircon U-Pb dating of granitoids yielded ages of around 326 ± 5 Ma, representing the first recovery of Late Paleozoic magmatic rocks from the Maya block, which could be genetically related to the convergence of Laurentia and Gondwana. The granitoids show the features of high K2O/Na2O, LaN/YbN and Sr/Y ratios, but very low Yb and Y contents, indicating an adakitic affinity. They are also characterized by slightly positive ԑNd(326Ma) of 0.17–0.68, intermediate initial 87Sr/86Sr(326Ma) of 0.7036–0.7047 and two–stage Nd model age (TDM2) of 1027–1069 Ma, which may indicate a less evolved crustal source. Thus, the adakitic granitoids were probably generated by partial melting of thickened crust, with source components similar to Neoproterozoic metagabbro in the Carolina block (Pan–African Orogeny materials) along Peri–Gondwana. Felsite dykes are shoshonitic with typical continental arc features that are sourced from a metasomatic mantle wedge by slab–fluids. Dolerite dykes display OIB–type features such as positive Nb and Ta anomalies and low ThNpm/NbNpm. In our interpretation, the Chicxulub adakitic granitoids of this study are formed by crustal anatexis due to asthenospheric upwelling resulting from slab breakoff. Through comparing sources and processes of Late Paleozoic magmatism along the Peri–Gondwanan realm, a tearing slab breakoff model may explain the discontinuous magmatism that appears to have occurred during the convergence of Laurentia and Gondwana.

Geology, Geochemistry and Radioactivity of Granitic and Volcanic Rocks at Hadarba Area, South Eastern Desert, Egypt

The present work deals with geology, geochemistry, radioactivity and tectonic environment of the granitic and volcanic rocks at Hadarba area, South Eastern Desert of Egypt. The granitic rocks comprise tonalite-granodiorite and monzogranite, while volcanic rocks include rhyolite, rhyodacite anddacite. These rocks are characterized by high concentrations of SiO2, Na2O, Fe2O3, K2O, Zr, Nb and Y but low in MgO, CaO, Cr, Ni, Sr, Ga and V. Field studies indicate that Dokhan volcanics extrude both tonalite-granodiorite and monzogranite with sharp contact forming a thick successive sequence of laminated acid lava flows, crystal lapilli tuffs and agglomerates. They range in composition from rhyolite, rhyodacite to dacite. Shear zone comprise lines–arranged intrusions trending NE-SW direction were recorded north Gabal Hadarba and extends for about 2.3 km in length with a width reaches up to 10 meter. These lines–arranged intrusions include quartz vein, microgranite and basic dykes. These shear zones cut through monzogranite. Petrochemical studies and tectonic discrimination diagrams for the monzogranite reveal that it is classified as granite developed in the within-plate tectonic environment, while Dokhan volcanics are classified as rhyolite and dacite developed in the immature island arcs and active continental margin environments.Field radiometric measurements of these granitic and volcanic rocks reveal low uranium and thorium contents. Uranium contents range from 1.2 to 2.7 ppm in the tonalite-granodiorite, from 1.1 to 8.5 ppm in the monzogranite, from 1.3 to 9.3 ppm in the Dokhan volcanic and from 1.4 ppm to 15.4 ppm in the felsite dyke.

Petrological and Geochemical Studies of Felsites around Mandya District, Western Dharwar Craton, South India, Karnataka

The Dharwar Craton in south India offers unique opportunity to study the natural cross-section of late Archaean continental crust. A preliminary attempt has been made in this study to understand the petrological and geochemical aspects of felsite dykes that are spread around Mandya district in south India. Geologically investigated area is made up of varied assemblages of rock types ranging from gneiss, ancient supracrustals of Sargur group, Peninsular gneiss, ultramafics, meta-ultramafites, amygdaloidal metabasalt and pillowed/BIF, granodorite, amphibolite and to younger intrusives of Neo-proterozoic age. Felsites in alternation with pegmatites are also exposed. Quartzo-feldspathic veins traverses the felsites at places and felsite with large phenocryst of feldspar are also found. At places felsites are highly jointed and the joint plane trend in E-W direction. Petrographic study shows that felsites are aphanitic and porphyritc in texture. The field and petrographic studies coupled with chemistry data suggest that the felsites dykes of the study area have evolved from a complex crystallization history and are formed in a volcanic arc granitic environment. The concentration of uranium up to 7.1 ppm indicates the potentiality of felsites for the occurrence of radioactive elements.

GEOLOGY, GEOCHEMISTRY AND RADIOACTIVITY OF GRANITIC AND VOLCANIC ROCKS ATHADARBA AREA, SOUTH EASTERN DESERT, EGYPT

The present work deals with geology, geochemistry, radioactivity and tectonic environment of the granitic and volcanic rocks at Hadarba area, South Eastern Desert of Egypt. The granitic rocks comprise tonalite-granodiorite and monzogranite, while the volcanic rocks include rhyolite, rhyodacite and dacite. These rocks are characterized by high concentrations of SiO2, Na2O, Fe2O3, K2O, Zr, Nb and Y but low in MgO, CaO, Cr, Ni, Sr, Ga and V. Field studies indicate that Dokhan volcanics extrude both tonalite-granodiorite and monzogranite with sharp contact forming a thick successive sequence of laminated acid lava flows, crystal lapilli tuffs and agglomerates. They range in composition from rhyolite, rhyodacite to dacite. Shear zone comprise lines–arranged intrusions trending NE-SW direction were recorded north Gabal Hadarba and extends for about 2.3 km in length with a width reaches up to 10 meter. These lines–arranged intrusions include quartz vein, microgranite and basic dykes. These shear zones cut through monzogranite.Petrochemical studies and tectonic discrimination diagrams for the monzogranite reveal that it is classified as granite developed in the within-plate tectonic environment, while Dokhan volcanics are classified as rhyolite and dacite developed in the immature island arcs and active continental margin environments. Field radiometric measurements of the granitic and volcanic rocks reveal low uranium and thorium contents. Equivalent uranium content ranges from 1.2 to 2.7 ppm in the tonalite-granodiorite, from 1.1 to 8.5 ppm in the monzogranite, from 1.3 to 9.3 ppm in the Dokhan volcanic and from 1.4 to 15.4 ppm in the felsite dyke.

Poster #T228 REDUCED ANTERIOR CINGULATE GRAY MATTER VOLUME AND THICKNESS IN SUBJECTS WITH DEFICIT SCHIZOPHRENIA

The Uchkoshkon tin deposit is one of the largest deposits in the Sn–W Saryjaz ore district of Eastern Kyrgyzstan. The district coincides spatially with the Saryjaz syntaxis, which is the narrowest portion of the South Tien Shan collisional belt. The syntaxis is distinguished by collisional thrust tectonics and postcollisional A-type granite magmatism of Permian age. The Uchkoshkon deposit is in the exocontact zone of a 260-km2, 273–279 Ma, ilmenite series, high heat production, biotite granite pluton. A satellite granite stock (400 × 250 m) and an explosive breccia pipe (450 × 150 m), crosscutting the stock, are the central structural features of the deposit area. The main stock, accompanied by Li–F felsite dykes, controlled early-stage Mo–W and Sn–W greisen–stockwork mineralization that surrounds the apical portion of the stock. The breccia pipe postdates the emplacement of the Li–F felsite dykes and greisen–stockwork mineralization. The pipe is rooted in main pluton and has been traced by a borehole to a depth of 800 m. The breccia was injected by a swarm of tree-like branching dykes with diffuse contours consisting of K- and B-rich rhyolite porphyry cementing the breccia fragments. The main breccia body has been intruded by a much smaller (150 × 100 m) hydrothermal breccia pipe with quartz–tourmaline matrix, which immediately preceded main-stage lode mineralization. The majority of the lodes occur outside the breccia pipe within a zone of pervasive scapolitization that surrounds the main pluton. The main-stage lodes are represented by quartz–tourmaline–cassiterite veins with minor sulfides, which occupy a series of steeply-dipping, near-parallel, north–northeast-trending shear fractures that are related to postcollisional strike-slip tectonics. Where quartz–tourmaline–cassiterite veins crosscut moderately-dipping skarn deposits, the latter are commercially mineralized with cassiterite, nordenskioldine and borosilicate minerals. The formation of both explosive breccia and main-stage lode mineralization was related to the evolution of B- and H2O-rich residual magma deep within the main pluton. The suggested formation model of the Uchkoshkon tin deposit is generally applicable to other major Sn- and Sn–W deposits in the Saryjaz ore district.

경사계를 이용한 대구 서부지역 반야월층 굴착 지반의 변위 분석

대구 지하철 굴착공사 중 노출된 굴착벽의 수평변위분석을 위해 서로 인접한 3개소의 경사계 계측자료를 이용하여 굴착지반의 변위특성 및 원인을 지질공학적 관점에서 고찰하였다. 연구지역은 경상누층군 하양층군 반야월층 지반에 해당하며 안산암질화산암, 석회질셰일, 사암, 호온펠스, 규장암맥 등으로 구성된 하부의 암반층과 이를 부정합으로 피복하고 있는 상부의 토사층으로 구성된다. 경사계 계측공 중 D4 지반의 암반층은 RMR V 등급이며, 층리면과 단층면을 따라 심도 12 m 지점에서 N34W 방향으로 최대수평변위량이 101.39 mm로 분석 되었고, D5 지반의 암반층은 RMR IV 등급이며, 셰일의 층리면, 셰일과 규장암의 접촉면을 따라 심도 9 m와 14 m에서 거의 남쪽 방향으로 최대수평변위량이 53.01 mm ~ 55.17 mm로 측정 되었다. Y6 지반의 암반층은 RMR III 등급이며, 상부 토사층과 하부 암반층의 경계면인 부정합면을 따라 심도 7 m 지점에서 S52W 방향으로 12.65 mm의 최대수평변위량을 나타낸다. 굴착벽에서 측정한 암반 내 불연속면들을 평사투영하여 예상되는 변위방향과 각 경사계 계측분석 결과 얻어진 수평변위방향이 거의 일치하는 결과를 보였으며, 굴착벽의 지중수평변위는 암반 내 불연속면의 발달정도와 종류, 배향 및 암석의 종류에 좌우되며, 굴착벽에 수직방향과 수평평행방향의 벡터 합성 방향으로 많이 발생한다. 또한 토사층 내 지중수평변위의 양상이 심도에 따라 비교적 곡선이며 연속적 궤적을 보이는데 반해 암반층 내 지중수평변위의 양상은 직선적이고 불규칙적 궤적을 나타낸다. To analyze lateral displacement of excavation walls exposed during the construction of Subway Line 1 in the Daegu region, inclinometer measurement data for sites D4, D5, and Y6 are investigated from the perspective of engineering geology. The study area, in the Banyawol Formation, Hayang Group, Gyeongsang Supergroup, is in the lower part of bedrock of andesitic volcanics, calcareous shale, sandstone, hornfels, and felsite dykes that are unconformably overlain by soil. The rock mass around the D4 site is classified as RMR-V grade and the maximum lateral displacement of 101.39 mm, toward N34W, was measured at a bedding-parallel fault, at a depth of 12 m. The rock mass around the D5 site is classified as RMR-IV grade and the maximum lateral displacement of 55.17 mm, toward the south, was measured at a lithologic contact between shale and felsite, at a depth of 14 m. The rock mass around the Y6 site is classified as RMR-III grade and the maximum lateral displacement of 12.65 mm, toward S52W, was measured at an unconformity between the soil and underlying bedrocks, at a depth of 7 m. The directions of lateral displacement in the excavation walls are vector sums of the directions perpendicular to the excavation wall and horizontally parallel to the excavation wall. Lateral displacement graphs according to depth in the soil profile show curvilinear trajectories, whereas those in bedrock show straight and rapid-displacement trajectories.

Geology and mineralization of the Uchkoshkon tin deposit associated with a breccia pipe, Eastern Kyrgyzstan

The Uchkoshkon tin deposit is one of the largest deposits in the Sn–W Saryjaz ore district of Eastern Kyrgyzstan. The district coincides spatially with the Saryjaz syntaxis, which is the narrowest portion of the South Tien Shan collisional belt. The syntaxis is distinguished by collisional thrust tectonics and postcollisional A-type granite magmatism of Permian age. The Uchkoshkon deposit is in the exocontact zone of a 260-km 2, 273–279 Ma, ilmenite series, high heat production, biotite granite pluton. A satellite granite stock (400 × 250 m) and an explosive breccia pipe (450 × 150 m), crosscutting the stock, are the central structural features of the deposit area. The main stock, accompanied by Li–F felsite dykes, controlled early-stage Mo–W and Sn–W greisen–stockwork mineralization that surrounds the apical portion of the stock. The breccia pipe postdates the emplacement of the Li–F felsite dykes and greisen–stockwork mineralization. The pipe is rooted in main pluton and has been traced by a borehole to a depth of 800 m. The breccia was injected by a swarm of tree-like branching dykes with diffuse contours consisting of K- and B-rich rhyolite porphyry cementing the breccia fragments. The main breccia body has been intruded by a much smaller (150 × 100 m) hydrothermal breccia pipe with quartz–tourmaline matrix, which immediately preceded main-stage lode mineralization. The majority of the lodes occur outside the breccia pipe within a zone of pervasive scapolitization that surrounds the main pluton. The main-stage lodes are represented by quartz–tourmaline–cassiterite veins with minor sulfides, which occupy a series of steeply-dipping, near-parallel, north–northeast-trending shear fractures that are related to postcollisional strike-slip tectonics. Where quartz–tourmaline–cassiterite veins crosscut moderately-dipping skarn deposits, the latter are commercially mineralized with cassiterite, nordenskioldine and borosilicate minerals. The formation of both explosive breccia and main-stage lode mineralization was related to the evolution of B- and H 2O-rich residual magma deep within the main pluton. The suggested formation model of the Uchkoshkon tin deposit is generally applicable to other major Sn- and Sn–W deposits in the Saryjaz ore district.

EMPLACEMENT CONDITIONS OF A PORPHYRITIC FELSITE DYKE AND TIMING OF MOTION ALONG THE COOLIN FAULT AT BEN LEVY, CO. GALWAY

A NNE-trending porphyritic felsite dyke located on Ben Levy in Co. Galway cross-cuts the E—W trending Coolin Fault. The porphyritic felsite is an altered rhyolite containing ocellar feldspar with rare apatite inclusions, quartz phenocrysts as well as minor ilmenite, rutile and zircon, all within a ground-mass of feldspar and quartz. High whole-rock K/Na and Rb/Sr ratios and textural evidence from cathodoluminescence (CL) imaging indicate that the dyke underwent hydrothermal alteration; however, immobile Zr vs TiO 2 for the Ben Levy Dyke is consistent with the trend of values for Galway Batholith dykes. Magmatic crystallization temperatures were calculated as 1067±14°C using Ti-in-zircon thermometry. Quartz phenocrysts may have been affected by diffusive loss of Ti post-emplacement, and record an average temperature of <574°C based on Ti-in-quartz thermometry. An age of 373.9±4.0Ma was determined from laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) U—Pb isotopic analysis of zircon. The Ben Levy porphyritic felsite is similar in age and orientation to other late-stage dykes which cross-cut the Galway Batholith to the south, and may represent the northernmost extension of late-stage dyke activity. Motion along the Coolin Fault occurred during the Caledonian Orogeny, between deposition of the Lough Kilbride Formation (436—428.2Ma) and intrusion of the Ben Levy Dyke at 373.9±4.0Ma.

EMPLACEMENT CONDITIONS OF A PORPHYRITIC FELSITE DYKE AND TIMING OF MOTION ALONG THE COOLIN FAULT AT BEN LEVY, CO. GALWAY

A NNE-trending porphyritic felsite dyke located on Ben Levy in Co. Galway cross-cuts the E-W trending Coolin Fault. The porphyritic felsite is an altered rhyolite containing ocellar feldspar with rare apatite inclusions, quartz phenocrysts as well as minor ilmenite, rutile and zircon, all within a ground-mass of feldspar and quartz. High whole-rock K/Na and Rb/Sr ratios and textural evidence from cathodoluminescence (CL) imaging indicate that the dyke underwent hydrothermal alteration; however, immobile Zr vs Ti02 for the Ben Levy Dyke is consistent with the trend of values for Galway Batholith dykes. Magmatic crystallization temperatures were calculated as 1067 + 14°C using Ti-in-zircon thermo metry. Quartz phenocrysts may have been affected by diffusive loss of Ti post-emplacement, and record an average temperature of < 574CC based on Ti-in-quartz thermometry. An age of 373.9±4.0Ma was determined from laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) U-Pb isotopic analysis of zircon. The Ben Levy porphyritic felsite is similar in age and orientation to other late-stage dykes which cross-cut the Galway Batholith to the south, and may represent the northernmost extension of late-stage dyke activity. Motion along the Coolin Fault occurred during the Caledonian Orogeny, between deposition of the Lough Kilbride Formation (436 -428.2Ma) and intrusion of the Ben Levy Dyke at 373.9 ±4.0Ma. Introduction Coolin Fault. The Coolin Fault lies within a thr ust belt associated with the Caledonian Orogeny A felsic porphyritic dyke outcrops along the foot(Phillips 2001). Based upon field relationships in this wall scarp of a NNE-trending normal fault, location, motion along the Coolin Fault must have which transects Ben Levy near Ballard, Co. Galway occurred after deposition of the Kilbride Formation. (Fig. 1). The dyke has a strike of ~040°, dips steeply The Kilbride is constrained to the Telychian Stage to the W, and is about 2-3 metres wide in locations (436^428.2Ma) of the Llandovery Series (Holland where contacts can be clearly seen on both sides 1985; Cocks 1989) by the presence of the brachiopod of the porphyritic felsite (Locations A and C). The Eocoelia curtisi curtisi within the Annelid Grits porphyritic felsite extends through the Dalradian member in the lower Kilbride (Harper et al. 1995). Ben Levy Formation and into the Lower Silurian In this study, the timing of motion along the Lough Mask and Kilbride Formations on the north Coolin Fault is constrained using U-Pb age dat side of Ben Levy, cross-cutting the E-W-trending ing. Cathodoluminescence (CL) imaging, major and Irish Journal of Earth Sciences 29 (2011), 1-13 © 2011 Royal Irish Academy doi: 10.3318/IJES.2011.29.1 This content downloaded from 157.55.39.217 on Mon, 18 Apr 2016 08:03:01 UTC All use subject to http://about.jstor.org/terms 2 Irish Journal of Earth Sciences (2011 ) Ordovician-Silurian Sediments and Volcanics Carboniferous Oughterard Granite p Connemara Metamorphic Complex Galway Batholith

The relationship between forceful and passive emplacement: The interplay between tectonic strain and magma supply in the Rosses Granitic Complex, NW Ireland

The Rosses Granitic Complex, NW Ireland, part of the late Caledonian (c. 400 Ma) Donegal Batholith, provides the opportunity to study the interplay between relative tectonic strain and magma supply rates in the wall of a major tectonic shear zone. Anisotropy of magnetic susceptibility (AMS) measurements, structural analysis and examination of key intrusive relationships were used to assess the accommodation of magma, associated deformation and magma flow pathways in this complex. In this case the varying emplacement styles, switching from forceful to passive, indicate that relative tectonic strain and magma supply rates were not constant. In the earliest component (a suite of microgranite sheets), AMS reveals subtle fabrics discordant to the sheet margins and is interpreted as post-emplacement deformation fabrics. The concordance of these fabrics to the next component of the complex, the main pluton (G1 and G2 monzogranite), indicates that this deformation was caused by the forceful emplacement of the pluton. AMS fabrics in G1 and G2 reveal a dome shaped foliation with an east-west lineation, indicating an east-west oriented magma transport direction. Outcrops of small stocks or cupolas similar to G2, east of the main pluton, link it to similar lithologies in the Main Donegal Granite further to the east via a partially exposed lateral feeder. This suggests east to west emplacement. The next component, a suite of subvertical porphyritic felsite dykes, is shown (from AMS and visible shearing fabrics) to have been emplaced passively under east-west tension. The final component comprises G3 and G4 of the main pluton, which passively cut all earlier components and contains significant amounts of aplite, pegmatite and greisen. These are interpreted to be cupolas or stocks emanating from an unexposed sheet probably similar in composition and mode of emplacement to G1 and G2. Thus, a general model is put forward where initially forceful subhorizontal sheets extending laterally from the nearby Main Donegal Granite shear zone, gave way to passive emplacement where the roofs of these and subjacent sheets failed. The control on siting of the complex can be related to lower crustal structures, which control upper crustal shear zones. The reason for the switch in emplacement style is suggested to be due to a waning magma supply, in a tectonic strain field where the extensional component was oriented east-west. The waning magma supply then possibly due to switching of the site of emplacement of magma, supplied by the Main Donegal Granite shear zone, to another member of the Donegal Batholith – the Trawenagh Bay Granite.

Neoproterozoic dextral faulting on the Najd Fault System, Saudi Arabia, preceded sinistral faulting and escape tectonics related to closure of the Mozambique Ocean

Abstract The Neoproterozoic Najd Fault System extends for 2000km across the East African Orogen, yet its history of motion and tectonic significance are widely debated. The Halaban-Zarghat Fault is the northeastern-most of the major NW-striking Najd faults in the Arabian Shield. Several sedimentary basins of the Neoproterozoic Jibalah Group are bounded by strands of the Halaban-Zarghat Fault and other Najd faults, particularly along right steps in the fault trace. Among the largest of the basins is the Jifn. The geometry of the Jifn Basin and the sedimentary facies of Jibalah Group indicate that it is a dextral pull-apart basin between strands of the Halaban-Zarghat Fault. A zone of high-grade mylonitic gneiss is located along a left step in the fault zone and may be a deeply eroded pop-up structure related to dextral transpression. Analysis of structural data from around and within the Jifn Basin, the position of other pull-apart basins and high-grade mylonite zones along the Halaban-Zarghat Fault are all consistent with early dextral movement along the Halaban-Zarghat Fault. Offsets of distinctive older rock units and transection of the Jifn Basin by sinistral faults, however, show that the latest and most significant sense of offset on the Halaban-Zarghat Fault and other Najd faults was sinistral. A U-Pb zircon date of 624.9 ± 4.2 Ma from rhyolitic basement of the Jifn Basin gives a lower limit for the formation of the basin and initiation of dextral movement along the Halaban-Zarghat Fault. This age is interpreted as the earliest age for the collision of East and West Gondwana. A 621 ± 7 Ma pluton is offset 10 km dextrally along the Halaban-Zarghat Fault, showing that dextral motions continued for some time past 621 Ma, before switching to sinistral motions, and accreted terranes caught between the two continents were forced toward an oceanic-free face to the north. A 576.6 ± 5.3 Ma U-Pb zircon date from an undeformed felsite dyke that intrudes the Jibalah Group gives an upper time limit for movement along the Halaban-Zarghat Fault. This may mark the time that collision and escape tectonics ended, or it may reflect the time that displacements were transferred to other Najd faults in more interior parts of the East African Orogen.

K-Ar ages of fault gouges and felsite dykes from the Median Tectonic Line in Chubu Region, central Japan

K-Ar ages of fault gouges and felsite dykes from the Median Tectonic Line in Chubu Region, central Japan

THE BORROWDALE VOLCANIC SERIES BETWEEN BUTTERMERE AND WASDALE, CUMBERLAND

Summary The lowest 3300 feet of the Borrowdale Volcanic Series are exposed adjacent to the eastern edge of the Ennerdale Granophyre between Wastwater and Buttermere. The succession comprises a lower Andesite Group of at least 3000 feet and an upper Dacite Group of more than 300 feet which in one area are contiguous with groups of andesites and rhyolites respectively, the main outcrops of which are west of the granophyre. The area has been subjected to earth movements of pre-Bala and Devonian age. The regional dip of the volcanic rock is between east and south-east. A thrust fault of the same age as the Devonian folds now separates the Skiddaw Slates and the Borrowdale Volcanic Series. Several high-angled faults of later, but probably still Devonian, age cross the area and trend between north-west and NNE. The Ennerdale Granophyre and associated felsite dykes have been intruded into the Borrowdale Volcanic Series. The emplacement of these intrusions, and the Eskdale Granite to the south, has been controlled by the structure of the Borrowdale Series.

Some Investigations of the Roraima Formation Intrusives in British Guiana

AbstractA traverse of a noritic gabbro sill (c. 1,000 feet thick) intruding the Roraima Formation showed no cryptic layering, but small-scale fractionation throughout the sill is indicated by the strongly-zoned plagioclase (An80→40), and orthopyroxene (En79→54) with outer rims of inverted pigeonite. Olivine (Fo80) is only present in the chilled margin.New evidence on the age of the granites underlying the Roraima Formation is given by the discovery of post-Roraima felsite dykes, with high-quartz phenocrysts.

XIII. The Igneous Geology of the Cumbrae Islands, Firth of Clyde

Introduction. The picturesque and conspicuous Cumbrae Islands, in the Firth of Clyde, have often been mentioned in geological literature, and many of their geological features and a few of their igneous rocks described; but there has never been a comprehensive statement of the characters and relations of the igneous suite as a whole in connection with the great Clyde Carboniferous lava-plateau of which it forms a part. Sir A. Geikie (1897, pp. 368-9, 385, 407-8) makes repeated references to the igneous phenomena displayed in these islands, and gives an excellent sketch of the Little Cumbrae, showing its fine regular lava escarpments. In regard to the characters and ages of the several groups of dykes (and sills) in the Great Cumbrae he follows the work of W. Gunn (1903), which was published later in a Survey memoir. Gunn distinguished four groups of dykes—(1) Trachyte (and bostonite), having a general E.N.E. direction; (2) porphyritic basalt, trending in the same direction; (3) E.W. dolerite dykes; and (4) N.N.W. dykes, probably of Tertiary age (Gunn, 1903, pp. 61-2). These are arranged in order of age, but we shall have reason later to question the general priority of the trachytes and bostonites. A few petrographical details are given in an appendix to the memoir (pp. 177-8). In these Transactions Mr. James Neilson has contributed a short paper on the Little Cumbrae and its minerals (1893), and Mr. John Smith has described the remarkable flow structure in a felsite dyke north of Downcraig Ferry in the This 250-word extract was created in the absence of an abstract

IV. The Pitchstones of South Arran

The pitchstones of Arran have been so well known for more than a century that further description may seem to require an apology. Except for Judd's description of the Drumadoon and Cir Mhor occurrences,1 most of the previous investigations have dealt with the Corriegills rocks. The object of the present paper is to give a description of the glassy rocks and the felsites associated with them, which are found outwith the area treated in the Geological Survey memoir on North Arran, and to discuss shortly several of the more striking characteristics which they exhibit. The occurrences considered include the Monamore Glen sills and dykes, the Glen Ashdale composite dyke, the series of intrusions in the neighbourhood of Tighvein and the Burican pitchstone. The Monamore Pitchstones. The earliest notice of this series of intrusions occurs in Bryce's “Geology of Arran.” In the edition published in 1859 no mention is made of it, but in the third edition, published about 1865, he describes two beds of pitchstone in “Moneadmhor Glen,” associated with “claystones, hornstone, quartz-rock, and porcellanite.”2 Zirkel,3 in his account of his journeyings in the Western Isles, mentions incidentally, in his description of the Corriegills and Tormore dykes, the same two intrusions, and notices that they contain thicker spherulites than is usual in this type of rock, and small green hornblende prisms, while associated with them is a “hornsteinahnliche felsit.” Similarly, Allport4 refers to them, but gives no details. This series of intrusions comprises one felsite dyke and This 250-word extract was created in the absence of an abstract